Thick stacks of clay minerals indicate chemical alteration of thick stacks of rock by interaction with liquid water on ancient Mars. Aluminum clays overlying iron/magnesium clays here in the ancient terrains of Mawrth Vallis indicate a change in environmental conditions. Aluminum clays may form by near-surface leaching while iron/magnesium clays may form in the subsurface. The image is from the High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter. (References: Wray et al., 2008; Loizeau et al., 2010.)